In recent years, the demand for lead-free piezoelectric materials has been increasing, and thus, the research for potassium sodium niobate ((K,Na)NbO3 (hereinafter, also referred to as KNN)) has been gaining focus. The KNN has attracted attention because it has a relatively high Curie temperature and favorable piezoelectric characteristics in the lead-free piezoelectric materials.
Furthermore, the practical application of a piezoelectric element using a thin film piezoelectric material instead of a bulk piezoelectric material is increasing. An example of such piezoelectric element using a thin film piezoelectric material is a gyro sensor, a pressure sensor, a pulse wave sensor, a shock sensor as a piezoelectric sensor, or a microphone using a piezoelectric effect which converts a force applied to a piezoelectric layer to a voltage; or a hard disk drive head slider or an ink jet print head as a piezoelectric actuator using reverse piezoelectric effect in which the piezoelectric layer is deformed when a voltage is applied to the piezoelectric layer; or a speaker, a buzzer or a resonator using the same reverse piezoelectric effect.
When the piezoelectric material is thinned, since the element can be miniaturized, applicable fields are widened and a large number of elements can also be manufactured in a mass on a substrate, mass productivity is increased. In addition, when used in sensors, many benefits such as an improvement in performance of sensitivity are provided.                [PTL 1] Japanese Patent No. 4588807        [PTL 2] Japanese Unexamined Patent Application Publication No. 2009-130182        [PTL 3] Japanese Unexamined Patent Application Publication No. 2008-192868        [PTL 4] International Publication WO 2003/070641        [NPL 1] Lee et al: Current Applied Physics 11 (2011) 5266        [NPL 2] Wang et al: Applied Physics Letters 97, 072902 (2010)        [NPL 3] Abazari et al: Appl. Phys. Lett. 92, 212903 (2008)        